This invention relates generally to solid-oxide fuel cells (SOFCs), and more particularly to systems and methods for boosting overall performance of a solid-oxide fuel cell (SOFC) while simultaneously separating a nearly pure stream of CO2 for sequestration or for use in generating electrical power to further increase overall efficiency of the process.
Fuel cells are electrochemical energy conversion devices that have demonstrated a potential for relatively high efficiency and low pollution in power generation. A fuel cell generally provides a direct current (dc) which may be converted to alternating current (ac) via for example, an inverter. The dc or ac voltage can be used to power motors, lights, and any number of electrical devices and systems. Fuel cells may operate in stationary, semi-stationary, or portable applications. Certain fuel cells, such as solid oxide fuel cells (SOFCs), may operate in large-scale power systems that provide electricity to satisfy industrial and municipal needs. Others may be useful for smaller portable applications such as for example, powering cars.
A fuel cell produces electricity by electrochemically combining a fuel and an oxidant across an ionic conducting layer. This ionic conducting layer, also labeled the electrolyte of the fuel cell, may be a liquid or solid. Common types of fuel cells include phosphoric acid (PAFC), molten carbonate (MCFC), proton exchange membrane (PEMFC), and solid oxide (SOFC), all generally named after their electrolytes. In practice, fuel cells are typically amassed in electrical series in an assembly of fuel cells to produce power at useful voltages or currents. Therefore, interconnect structures may be used to connect or couple adjacent fuel cells in series or parallel.
In general, components of a fuel cell include the electrolyte and two electrodes. The reactions that produce electricity generally take place at the electrodes where catalyst is typically disposed to speed the reactions. The electrodes may be constructed as channels, porous layers, and the like, to increase the surface area for the chemical reactions to occur. The electrode that electrochemically reduces oxygen (typically from air) is referred to as the cathode, while the electrode that electrochemically oxidizes fuel is referred to as the anode. The electrolyte carries electrically charged particles from one electrode to the other and is otherwise substantially impermeable to both fuel and oxidant. In the case of a Solid Oxide Fuel Cell (SOFC), the electrolyte is a solid ceramic oxide that conducts negative oxygen ions at sufficiency high temperature (typically above 500 C). Because the electrolyte in an SOFC is only conductive at high temperature, the anode fuel inlet stream and the cathode oxidizer inlet stream must typically be preheated to high temperature (typically above 500 C). This preheating is typically accomplished via recuperative heat exchange with the hot fuel cell exhaust.
Typically, the fuel cell converts hydrogen (fuel) and oxygen (oxidant) into water (byproduct) to produce electricity. The byproduct water may exit the fuel cell as steam in high-temperature operations. This discharged steam (and other hot exhaust components) may be utilized in turbines and other applications to generate additional electricity or power, providing increased efficiency of power generation. If air is employed as the oxidant, the nitrogen in the air is substantially inert and typically passes through the fuel cell. Hydrogen fuel may be provided via local reforming (e.g., on-site steam reforming) of carbon-based feedstocks, such as reforming of the more readily available natural gas and other hydrocarbon fuels and feedstocks. Examples of hydrocarbon fuels include natural gas, methane, ethane, propane, methanol, syngas, and other hydrocarbons. The reforming of hydrocarbon fuel to produce hydrogen to feed the electrochemical reaction may be incorporated with the operation of the fuel cell. Moreover, such reforming may occur internal and/or external to the fuel cell. For reforming of hydrocarbons performed external to the fuel cell, the associated external reformer may be positioned remote from or adjacent to the fuel cell.
Fuel cell systems that can reform hydrocarbon internal and/or adjacent to the fuel cell may offer advantages, such as simplicity in design and operation. For example, the steam reforming reaction of hydrocarbons is typically endothermic, and therefore, internal reforming within the fuel cell or external reforming in an adjacent reformer may utilize the heat generated by the typically exothermic electrochemical reactions of the fuel cell. Furthermore, catalysts active in the electrochemical reaction of hydrogen and oxygen within the fuel cell to produce electricity may also facilitate internal reforming of hydrocarbon fuels. In SOFCs, for example, if nickel catalyst is disposed at an electrode (e.g., anode) to sustain the electrochemical reaction, the active nickel catalyst may also reform hydrocarbon fuel into hydrogen and carbon monoxide (CO). Moreover, both hydrogen and CO may be produced when reforming hydrocarbon feedstock. Thus, fuel cells, such as SOFCs, that can utilize CO as fuel (in addition to hydrogen) are generally more attractive candidates for utilizing reformed hydrocarbon and for internal and/or adjacent reforming of hydrocarbon fuel.
In general, high operating temperatures within the fuel cell and the presence of the byproduct steam generally promote internal or adjacent reforming of hydrocarbon. Advantageously, excess steam in the fuel cell may reduce the deposition of elemental carbon within the fuel cell and in adjacent reformers. On the whole, internal and/or adjacent reforming and their integration with the fuel cell operation may improve the efficiency and/or economics of the fuel cell operations.
Unfortunately, it is generally difficult to maintain a sufficiently high steam-to-carbon ratio in all regions of a fuel cell to prevent the formation of elemental carbon and the associated carbon deposition, especially if internal reforming is intended to occur on an electrode (e.g., anode) along with for example, the electrochemical reactions. The fuel cell region near the inlet is especially vulnerable to carbon formation. That is, the entering fuel to be internally steam reformed is starved for steam or liquid water (H2O) due to the typical gradient of increasing H2O (e.g., steam) concentration from the inlet to the outlet of the fuel cell. The H2O concentration generally increases in the direction of fuel flow toward the exit, and therefore, excess H2O is generally present at the exit region of the fuel cell. Carbon is generally expected to form near the cell inlet because the H2O concentration is lowest at the inlet. Carbon deposition in the fuel cell can lead to poor heat/mass transfer, damage, and/or failure of the fuel cells.
Sustaining long term operation of fuel cells may be problematic due to accumulation of carbon deposits within the fuel cell. Such carbon deposits are typically relatively worse if fuel cells rely on carbon-based feedstocks instead of cleaner hydrogen-based feedstocks. Eventually, the fuel cell may need to be shut down or replaced, interrupting production of electricity and increasing maintenance costs of the fuel cell system, for example. Further, reformers or pre-reformers used for external reforming and/or adjacent reforming may also be subjected to significant carbon deposition. Therefore, these reformers are also generally shut down for regeneration (e.g., via steam), increasing operating and maintenance costs, and reducing the efficiency of the overall fuel cell system.
The anode unit of an SOFC typically electrochemically oxidizes at most 80% of fuel into reaction products, the remaining 20% passing unoxidized into the exhaust. The upper limit on utilization is due to high concentrations of reaction products which hinder the chemical reaction near the downstream end of the anode and can cause damage to the fuel cell material.
In view of the foregoing, there is a need to provide a technique that increases the fuel utilization of an SOFC. It would be further beneficial if the technique were to achieve separation of CO2 from the fuel stream for sequestration or for expansion through a turbine, for example, to generate electric power thus increasing overall efficiency of the process.